Stepper motor tracking accuracy control for spectrophotometers

ABSTRACT

A tracking accuracy control system for a spectrophotometer utilizing stepper motors for controlling the wavelength drive and chart drive system. An exact ratio between the wavelength drive motor and chart drive motor is maintained under variable speed conditions by utilizing an error signal which normally operates the reference beam attenuator, this error signal being transmitted through an absolute value circuit to produce a voltage whose magnitude is the absolute value of the error signal. This absolute value is then transmitted through a fast attack, slow release hold-over circuit to control a voltage controlled oscillator in such a way as to reduce wavelength scan speed in an amount proportional to the error while simultaneously controlling chart speed in an exact ratio to the scan speed. The output of the voltage controlled oscillator is a pulse train having a frequency proportional to the magnitude of the absolute value of error signal.

This is a continuation of application Ser. No. 433,396, filed Jan. 14,1974, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to analyzers of the double beam, optical nulltype such as spectrophotometers utilizing beam intensity balancing, andin particular, to spectrophotometers having recording chart drivesystems.

In analyzers of this type, radiation from a source is switched along areference beam path and a sample beam path, the two beams beingmodulated and recombined at a thermocouple or other electrical signalgenerator. A sample to be analyzed is placed in one path and an errorsignal is used to drive a servo which varies the intensity of the beamin the reference path to achieve a null or zero signal at thethermocouple. The servo position is a measure of the sample content.Ordinarily, the wavelength of the radiation source is scanned over arange by a monochromator during the measurement to provide a spectrummeasure of the sample. This spectrum is plotted on a chart which canhave either a fixed chart with a pen moving along an X-Y axis of thechart or a moving chart having the pen traversing one axis of the chartalong a fixed line.

In either type of recording under closed loop servo conditions thewavelength scan speed is controlled according to the speed of responseof the instrument dynamics which places the beam attenuator such as acomb into the reference beam path, thus resulting in variable speed ofthe wavelength scanner according to instrument dynamics. Since the chartor pen speed must be controlled in proportion to the wavelength scanspeed, prior art systems have utilized a single variable speed motorwith mechanical gearing to both the wavelength drive and chart drive tomaintain the ratio. Due to the lash or play in the mechanical gearingand further depending upon the amount of gearing necessary due tophysical placement of the wavelength drive and chart drive, precisetracking control is virtually impossible to obtain.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide a new andimproved spectrophotometer utilizing a voltage controlled oscillator todrive both a chart drive stepper motor and a wavelength scan speedstepper motor.

It is another object of this invention to provide a new and improvedtracking accuracy control utilizing a voltage controlled oscillator tosimultaneously control the wavelength motor through wavelength dividersand a chart motor through chart dividers to maintain an exact ratiobetween the speed under variable speed conditions.

The foregoing and other objects of the invention are accomplished byhaving an optical null spectrophotometer with a recording chartprint-out with both the wavelength motor and the chart drive motor beingstepper motors simultaneously controlled by a voltage controlledoscillator, the output of which provides a pulse train at a frequencydetermined by the voltage controlled oscillator input. The voltagecontrolled oscillator input is an absolute value error signal derivedfrom the error signal utilized to control the position of the referencebeam attenuator. The voltage controlled oscillator provides switchselectable fixed frequency in combination with independent voltagecontrolled frequency.

The novel features which are believed to be characteristic of theinvention are set forth with particularity in the specification whichcan best be understood by reference to the following description andaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a spectrophotometer utilizing a steppermotor tracking accuracy control;

FIG. 2 is a schematic diagram of the absolute value detector andhold-over circuit utilized in the system of FIG. 1; and

FIG. 3 is a schematic diagram of a voltage controlled oscillatorutilized in the system of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawing, and particularly to FIG. 1, the apparatusincludes a source 10, a beam switching system 11, a monochromator 12 anda thermocouple 13. The source 10 may be any suitable device whichproduces radiation over the spectrum being analyzed. The beam switchingsystem includes half mirrors 14, 15 which are rotated in synchronism,and reflecting mirrors 16, 17, providing a sample beam path 18 and areference beam path 19. A sample cell 22 is positioned in the samplebeam path 18 for containing the sample to be analyzed. Means for varyingthe intensity of the beam along the reference beam path 19 is positionedtherein. A typical example is the comb 23 which is driven into and outof the reference beam path by a motor 24.

The monochromator 12 includes means for dispersing the beam passingtherethrough, shown here as a prism 27, and a slit 28 which permits onlya small fraction of the dispersed beam to impinge on the thermocouple13. The prism 27 is rotated by a scan motor 29 during the analysis toscan the entire spectrum of interest past the slit 28. The scan motor 29employed in this embodiment is a stepper motor which is rotated by meansof discrete digital pulses, the speed being dependent on the frequencyof the pulses. The motor 29 is energized from a wavelength motor driver30 which is ordinarily set to operate the motor at a constant rate. Aprogrammed change in scan speed over the spectrum is usually desired andis conventionally accomplished by coupling the motor to the prism orother dispersing element by means of a cam of appropriate contour.

The thermocouple 13 produces an electrical error signal proportional tothe difference in intensity of the beams traversing the sample path andreference path with the error signal cyclically varying at the beamswitching rate, which ordinarily is in the range of 5 to 20 cycles persecond.

The error signal from the thermocouple 13 is connected to an amplifier33, a demodulator 34, a period circuit 35 and an amplifier 36. With theexception of the stepper motor being employed as the scan motor 29 thesystem thus far described is conventional. A system of this type isshown and described in U.S. Pat. No. 3,176,576. The output of theamplifier 36 drives the comb motor 24, with the amplifier and motorfunctioning as the comb servo. The demodulator 34 is operated insynchronism with the beam switching system and converts the a.c. errorsignal to d.c. Various types of demodulators may be used with themechanical chopper or switch beam preferred at the relatively lowfrequencies ordinarily encountered in such instruments. The periodcircuit 35 is a low pass filter that limits the response rate of thecomb servo and reduces the sensitivity of the instrument to sharptransients in the error signal such as are ordinarily produced by noise.The time constant of the period circuit is selected as a compromisebetween the maximum response rate of the comb servo and the acceptablenoise level and typically is in the range of 1/4 to 16 seconds. Thesimplest form of period circuit which is used in many instruments, is aresistance-capacitance filter section comprising a series resistor and ashunt capacitor.

It would be desirable in the operation of spectrophotometers to have thescan motor 29 run at a high speed and to omit the period circuit so thata complete spectrum analysis could be completed in a relatively shorttime with a high degree of accuracy in the recorded output. However, thepresence of noise ordinarily requires the period circuit, which reducesthe response rate of the comb servo. Then when large error signals aregenerated the comb servo lags behind the scan system and errors occur inthe recorded output, these errors ordinarily being identified as errorsin tracking accuracy. One method of improving the tracking accuracy isto reduce or suppress the scan motor speed during the existence of largeerror signals so that the comb servo with its limited response rate canaccurately follow the error signals. It has also been found that thetracking accuracy can also be improved by controlling the time constantof a period circuit with the time constant being reduced duringconditions of large error signals so that the response rate of the combservo increases and permits rapid and accurate operation of the nullingsystem. The particular method of tracking accuracy control heretoforeemployed was dependent upon the scanning speed of the instrument. At arelatively high scanning speed the speed suppression or reductionapproach for improving the tracking accuracy was preferred since, athigh scanning speeds, the time constant of the period circuit will berelatively short. Alternatively, with the instrument being operated at arelatively low speed and with a relatively long period or large timeconstant, the period suppression or time constant control was preferredso as not to unduly extend the analysis time.

The diagram of FIG. 1 shows a system which utilizes scan speedsuppression operable over a wide range of scanning speeds. Thedemodulated error signal appearing at the output of demodulator 34 is acomb servo error signal which is a voltage whose magnitude and polarityare determined by the difference between the desired comb position andactual comb position. This output is transmitted into an absolute valuecircuit 38 to produce a voltage whose magnitude is determined by theabsolute value of the error signal. This value is ordinarily above theminimal acceptable error level, which minimal error level is necessaryfor proper functioning of a servo system. The output of absolute valuecircuit 38 is then transmitted into a hold-over circuit 40 which is afast attack slow release circuit, the attack time of which must be muchless than the instrument period and the release time of which should begreater than the instrument period. The output of the hold-over circuit40 controls the voltage controlled oscillator 42 in such a way as toreduce scan speed in an amount proportional to the error appearing atthe output of demodulator 34. By providing a sufficient dynamic rangefor the voltage controlled oscillator 42, the result is an instrumentwhose scan speed is controlled by the characteristics of the sample 22.This provides a spectrum analysis with both error and scan timeminimized for every sample.

The output of the voltage controlled oscillator 42 is a pulse train, thefrequency of which is proportional to the absolute value of the errorsignal. The output frequency is divided by switch selectable wavelengthdividers 44 to produce a pulse train of suitable frequency for thedesired scan speed to the wavelength motor driver 30 and to the chartdividers 46 which controls the chart motor driver 48 which in turndrives the chart stepper motor 50.

The chart frequency dividers 46 are similarly switch-selectable as arethe wavelength frequency dividers 44. While the stepper motor 50 isherein described as relating to the chart speed of a moving chartpassing a recording pen traversing a given line, it is also to beunderstood that the stepper motor 50 can be employed to move a penmechanism along one axis with respect to a fixed chart. In either eventthe principles remain the same, resulting in the elimination of anymechanical connection between wavelength and chart drives whilemaintaining the exact ratio between the speeds under variable speedconditions by use of the voltage controlled oscillator 42.

The switch selected wavelength dividers 44 and the switch selected chartdividers 46 can be manually selected by an operator to a desired scanspeed and chart speed based on the results anticipated of a givensample.

As previously stated, the output of the demodulator 34 is a comb servoerror signal having a magnitude and polarity determined by thedifference between the desired comb position and the actual combposition. This signal is then converted by the absolute value of circuit38 to an absolute magnitude in a conventional manner. The triggering ofthe absolute value circuit is set up at a minimal level of errordesired, and when this level is exceeded, the excess absolute valueerror signal is transmitted to the hold-over circuit 40.

In the physical apparatus of the embodiment the hold-over circuit iscombined with the absolute value circuit 38 and as shown in FIG. 2 theerror signal at input 50 is the output of demodulator 34. This signal isamplified in amplifier 52 and suitably transmitted over leads 54 to thenon-inverting input of operational amplifier 56 and through resistor 58to the inverting input of a second operational amplifier 60. Thenon-inverting input of operational amplifier 60 is grounded. Diodes 62and 64, respectively, are provided from the output to the invertinginputs of amplifiers 56 and 60, respectively. The output of amplifier 56is connected to the cathode of diode 68 which has the anode thereofconnected to an output lead 70, the lead 70 also being connected throughresistor 72 back to the inverting input of amplifier 56. Similarly,connected in circuit with amplifier 60 are diodes 74 and resistor 76.The lead 70 has one end thereof connected to a hold-over capacitor 78,the other end of which is connected to ground. The output lead 70 isconnected to the input of a buffer and level shifter circuit 80 ofconventional design to provide the output to the voltage controlledoscillator 42 on lead 82.

Depending on the polarity of the error signal appearing at input lead50, either one or the other of operational amplifiers 56 will provide acharge path for capacitor 78 through either diode 68 or 74,respectively, depending on which diode is forward biased. Due to the lowimpedance provided in the charge path by the diode, the capacitor 78will charge rapidly to provide a negative voltage on lead 70 which isalways at a zero or negative voltage value. In this manner an error isimmediately detected to charge the capacitor to slow the scan speed.Resistors 72 and 76 provide the discharge path for the capacitor 78,these resistors being high resistance values (approximately 100 K ohms).Once the capacitor is fully charged in accordance with the error signal,the operational amplifier is de-energized, thereby permitting thedischarge of capacitor 78 slowly through either resistors 72 or 76. Thisprovides the slow release previously referred to, to control the systemas zero error is approached. The output appearing on lead 82 istransferred to the voltage controlled oscillator 42.

As seen in FIG. 3, input leads 84 receive the output of the hold-overcircuit appearing on lead 82 (FIG. 2). This input is transferred throughresistor 86 to the gate terminal of a junction field-effect transistor87 which has a resistor 88 connected between the source and drainterminals thereof. The drain terminal is also coupled through resistor90 through resistor 92 to a positive source of voltage +V₁. Theintermediate terminal between resistors 90 and 92 is connected to theoutput of amplifier 94, the output also being coupled through feedbackresistor 96 to the noninverting terminal thereof. The noninvertingterminal of operational amplifier 94 is connected to the intermediatepoint between resistors 98 and 100, the other end of resistor 98 beingconnected to a positive voltage source +V₁ with the other end ofresistor 100 being connected to ground. The source terminal oftransistor 87 is connected to one end of a capacitor 102, the other endof which is coupled to ground, the source terminal also being coupled tothe source terminal of an insulated gate field-effect transistor 104 aswell as the inverting input of amplifier 94. Connected between thesource terminal of transistor 104 and ground is a series resistancecapacitance network consisting of resistor 106 and capacitor 108.Connected between the drain terminal of field-effect transistor 104 andground is a plurality of capacitors 110, 112, 114, 116 and 118 throughswitches 111, 113, 115, 117 and 119, respectively. In parallel withthese capacitors are series RC circuits through switches 120 and 122.The gate terminal of transistor 104 is connected to a source of voltage+V₁ and also through a normally open switch 124 which is activated by acam 126 which is part of the scan speed drive mechanism that rotatesprism 27 within monochromator 12.

The circuit of FIG. 3 is essentially an R-C circuit operationalamplifier relaxation oscillator with a manually selectable capacitanceand an error voltage selectable resistance. The scan speed is selectedby activating any of the switches 111, etc., to place in parallel anyone or more capacitors to thereby vary the capacitance overall and thusselect a scan speed. The plurality of capacitors in parallel can beselected to provide a wide dynamic range of operating speed forwavelength scanning. The output is a train of pulses appearing at theoutput of amplifier 94 at leads 130.

In normal operation, that is, with zero error signal, the field-effecttransistor 87 is biased "full on," thereby effectively rendering a shuntpath across resistor 88. The transistor 87 is a junction field-effecttransistor which is bidirectionally conductive, the conductivity beingdetermined by the gate electrode terminal voltage. The net effect is avoltage-controlled variable resistance in parallel with resistor 88.Briefly, the order of magnitude of resistance 88 determines the slowspeed of the pulse train output when the field-effect transistor 87 isnonconductive, while the resistive value of resistance 90 determines thefast speed within a selected scan speed range when the transistor 87 isfully conductive. For example, resistor 88 is in the order of magnitudeof 300,000 ohms while resistor 90 is in the order of magnitude of 1,000ohms.

The operational amplifier 94 is a high speed differential comparatorbiased as a relaxation oscillator. The capacitance of the frequencydetermining circuit of the multivibrator is switch selected bydepressing any one of the switches 111, etc., while the resistance valueof the RC circuit is determined by resistor 90 and by the conductivityof transistor 87 in parallel with resistor 88. Thus, voltage controlcapability is achieved by connecting a junction field-effect transistor87 in parallel with a portion of the resistance of the frequencydetermining circuit, making the total resistance a function of the gatevoltage on the field-effect transistor.

Resistors 98 and 100 provide a threshold voltage level for thenoninverting input of amplifier 94 while resistor 96 provides a feedbackloop between this input and the output.

Thus, it can be seen with a given selection of scan speed by activationof any of the switches 111, etc., a speed range is determined withcontrol within the range determined by the error voltage appearing atinputs 84 to the transistor 87. The transistor 104 is normally biased toconduction.

The wavelength scan in a spectrophotometer is usually controlled inconjunction with a wavelength cam coupled to the prism. A second cam 126is utilized and rotates with the wavelength cam, which may be shaped forthree order changes, for example. The second cam 126 is suitablydetented, as at portion 127 to coincide with the order changes. When thecam 126 which is coupled to the drive of the prism 27 reaches theportion 127 of the cam edge where it is desirable to increase thewavelength scan speed to maximum, the cam follower switch arm of switch124 is activated, thereby driving the gate of transistor 104 to groundresulting in transistor 104 becoming nonconductive and therebydisconnecting all the switch selectable capacitors. At this pointcapacitor 108 in series with resistor 106 determines the RC timeconstant of the relaxation oscillator circuit driving the clock output130 at maximum frequency to accelerate the wavelength scan speed. Thecircuit of FIG. 3 thus results in a voltage variable clock for a speedsuppression system using stepper motors.

The wavelength dividers 44 and chart dividers 46 are conventionalfrequency dividers switch selected to predetermined divider formats.

Although the wavelength divider 44 output is shown as being coupled tothe chart dividers 46, it is also possible to couple the chart divider46 to the output of the voltage controlled oscillator 42 to provide theflexibility required depending upon whether the chart is moving or thechart is stationary with the pen scanning the chart. In any event, whilethere has been shown and described a preferred embodiment, it is to beunderstood that various other adaptations and modifications may be madewithin the spirit and scope of the invention.

We claim:
 1. In a radiant energy analyzer having a radiant energysource, reference and sample radiant energy beam paths, a monochromatorand a wavelength scanning mechanism therefor, means generating an errorsignal which varies as a function of the difference in the intensity inthe reference and sample beam paths, attenuator means responsive to theerror signal to balance the energy in the beams, the improvementcomprising:a first stepper motor for driving the wavelength scanningmechanism to continuously vary the wavelength output of themonochromator; an oscillator having a pulse train output ofpredetermined frequency coupled to drive the first stepper motor, andhence the wavelength scanning mechanism, at a predetermined scan speed;means responsive to changes in the error signal for varying thefrequency of the pulses applied to the stepper motor, whereby the scanspeed of the wavelength scanning mechanism is decreased or increased fora corresponding increase or decrease in the error signal to improve thetracking accuracy between the wavelength scanning mechanism and theattenuator means; chart recording means for recording a radiant energyparameter of the sample and a second stepper motor for driving the chartrecording means along an axis thereof, the pulse train output of theoscillator being coupled to drive both the first and second steppermotors in a fixed speed relationship at speeds proportional to theoutput frequency of the oscillator; and first and second variablefrequency dividers and means coupling the oscillator pulse train outputthrough said first variable frequency divider to the first stepper motorand through a series connection of said first and second variablefrequency dividers to the second stepper motor.
 2. The analyzer of claim1 wherein the oscillator is a signal controlled, variable frequencyoscillator and the means responsive includes means coupling the errorsignal as a control input signal to the variable frequency oscillator,whereby the pulse train output frequency of the oscillator, and hencethe scan speed of the wavelength scanning mechanism and the speed of thechart recording means, is varied in response to changes in the errorsignal.
 3. In a radiant energy analyzer having a radiant energy source,reference and sample radiant energy beam paths, a monochromator and awavelength scanning mechanism therefor, chart recording means forrecording a radiant energy parameter of a sample, electrical signalgenerating means generating a d.c. error signal having an amplitudevarying as a function of the difference in the intensity in thereference and sample beam paths and a polarity indicative of therelative direction thereof, attenuator servo loop means responsive tosaid error signal to balance the energy in said beams, the improvementcomprising:a first stepper motor for driving said wavelength scanningmechanism to continuously vary the wavelength output of saidmonochromator; a second stepper motor for driving said chart recordingmeans along an axis thereof; a variable frequency oscillator having apulse train output coupled to drive both said first and second steppermotors in a fixed speed relationship at speeds proportional to theoutput frequency of said oscillator; circuit means responsive to saiderror signal for generating a control signal proportional to theabsolute value of said error signal, said circuit means including acapacitor across which said control signal is established and means forrapidly charging said capacitor responsive to the existence of saiderror signal and for slowly discharging said capacitor in the absence ofsaid error signal; and means coupling said control signal to saidvariable frequency oscillator in response to changes in said errorsignal whereby the speeds of the first and second stepper motors aredecreased or increased for a corresponding increase or decrease in saiderror signal.
 4. The analyzer of claim 3 wherein said means for rapidlycharging and slowly discharging said capacitor includes:first and secondoperational amplifiers each having inverting and non-inverting inputterminals and an output terminal; means for coupling the error signal tothe non-inverting input terminal of the first amplifier and to theinverting input terminal of the second amplifier; first and seconddiodes coupling the respective output terminals of the first and secondamplifiers to a first terminal of said capacitor, the first and seconddiodes completing respective first and second charge paths between thefirst and second amplifiers and the first terminal of said capacitor forcharging said capacitor in response to the error signal; the first andsecond diodes being poled in the same direction between the amplifierassociated with each and the first terminal of said capacitor, wherebyerror signals of a first polarity are translated exclusively by thefirst amplifier and associated diode and error signals of the oppositepolarity are inverted and translated exclusively by the second amplifierand associated diode to thereby develop the control signal across saidcapacitor proportional to the absolute value of the error signal; and aresistor coupled between the first terminal of the capacitor and theinverting input terminal of the second amplifier to provide a dischargepath for said capacitor.